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A Gold Exchange: A Mechanistic Study of a Reversible, Formal Ethylene Insertion into a Gold(III)–Oxygen Bond

Langseth, Eirin, Nova, Ainara, Tråseth, Eline Aa., Rise, Frode, Øien, Sigurd, Heyn, Richard H., Tilset, Mats
Journal of the American Chemical Society 2014 v.136 no.28 pp. 10104-10115
X-ray diffraction, ambient temperature, dissociation, ethylene, gold, isomers, ligands, methylene chloride, nuclear magnetic resonance spectroscopy, olefin, thermodynamics
The Au(III) complex Au(OAcF)₂(tpy) (1, OAcF = OCOCF₃; tpy = 2-p-tolylpyridine) undergoes reversible dissociation of the OAcF ligand trans to C, as seen by ¹⁹F NMR. In dichloromethane or trifluoroacetic acid (TFA), the reaction between 1 and ethylene produces Au(OAcF)(CH₂CH₂OAcF)(tpy) (2). The reaction is a formal insertion of the olefin into the Au–O bond trans to N. In TFA this reaction occurs in less than 5 min at ambient temperature, while 1 day is required in dichloromethane. In trifluoroethanol (TFE), Au(OAcF)(CH₂CH₂OCH₂CF₃)(tpy) (3) is formed as the major product. Both 2 and 3 have been characterized by X-ray crystallography. In TFA/TFE mixtures, 2 and 3 are in equilibrium with a slight thermodynamic preference for 2 over 3. Exposure of 2 to ethylene-d₄ in TFA caused exchange of ethylene-d₄ for ethylene at room temperature. The reaction of 1 with cis-1,2-dideuterioethylene furnished Au(OAcF)(threo-CHDCHDOAcF)(tpy), consistent with an overall anti addition to ethylene. DFT(PBE0-D3) calculations indicate that the first step of the formal insertion is an associative substitution of the OAcF trans to N by ethylene. Addition of free –OAcF to coordinated ethylene furnishes 2. While substitution of OAcF by ethylene trans to C has a lower barrier, the kinetic and thermodynamic preference of 2 over the isomer with CH₂CH₂OAcF trans to C accounts for the selective formation of 2. The DFT calculations suggest that the higher reaction rates observed in TFA and TFE compared with CH₂Cl₂ arise from stabilization of the –OAcF anion lost during the first reaction step.